Add a "check-for-deprecated-procedures-in-script-fu" make target that
looks for usage of deprecated procedures in *.scm files. We currently
use quite a bit of deprecated procedures. When we don't do that
anymore, we should connect this to make check somehow so our nightly
build fails if someone deprecated a procedures without also porting
all clients.
Run it like this from the source root:
make -C plug-ins/script-fu check-for-deprecated-procedures-in-script-fu

instead of always passing FALSE when changing to a different
layer. For some reason we were displaying the right state anyway,
which can only mean that we do things redundantly here and call this
function twice.